An X-ray imaging apparatus for rounds includes a cart having wheels in order to move among hospital rooms, and has a configuration adapted to irradiate a subject with X-rays that are emitted from an X-ray tube supported by a supporting member and of which the irradiation field is restricted by a collimator, and detect X-rays having passed through the subject by an X-ray detector such as a flat panel detector or an image intensifier (I.I.). Also, such an X-ray imaging apparatus for rounds is mounted with a battery in the cart in order to make it possible to perform X-ray imaging in an unspecified place including the case where no external power supply is provided (see Patent Literature 1 and Patent Literature 2).
Such a battery is a supply source of electric power for driving the wheels in order to move the X-ray imaging apparatus for rounds and electric power for emitting the X-rays from the X-ray tube. In addition, the electric power is preliminarily charged in the battery from an external power supply through a power supply cord and a charging circuit.
The charging circuit incorporates a charging current control transistor, and it is known that at the time of charging, the charging current control transistor produces heat. For this reason, for the purpose of reducing the temperature rise of the charging current control transistor, there is proposed a charging device adapted to, depending on the temperature of a charging current control transistor, be able to change a charging current value or a charging voltage value at the time of charging set for a charging circuit depending on a battery charging method (see Patent Literature 3).
Meanwhile, it is known that when supply voltage from an external power supply for charging a battery varies in excess of a predetermined voltage variation range, the variation interrupts the stable action of an apparatus. For this reason, for the X-ray imaging apparatus for rounds, an allowable voltage variation range is set in order to prevent the failure of the apparatus caused by a variation in supply voltage. In addition, the X-ray imaging apparatus for rounds includes a safety mechanism that when the supply voltage varies in excess of the set voltage variation range, detects the variation as an error and stops power supply from an external power supply.
FIG. 5 and FIG. 6 are graphs explaining the relationship between a variation in voltage and charging current immediately after the start of charging of a battery in a conventional X-ray imaging apparatus for rounds. In FIG. 5 and FIG. 6, the upper graphs illustrate the variations in the voltage (V) of an external power supply, and the lower graphs illustrate variations in the charging current (A). In addition, the horizontal axes of the graphs represent time (second). Note that the graphs illustrate cases when charging a lead-acid battery at a constant current.
In the conventional X-ray imaging apparatus for rounds, as the external power supply for charging the battery, a commercial power supply of which a variation in supply voltage is controlled to be within a predetermined range (e.g., a voltage variation range with respect to a standard voltage is ±10%) is assumed. For this reason, a voltage variation range ±X % (indicated by dashed lines in the graphs) allowable by the X-ray imaging apparatus for rounds is set to, for example, ±10 to 15%. In addition, the battery charging is adapted to be started, regardless of the variation in the power supply voltage, at a constant charging current value set for a charging circuit. Immediately after connecting the X-ray imaging apparatus for rounds to the external power supply, inrush current flows to temporarily cause a voltage drop. Unless such a temporal voltage drop exceeds the voltage variation range ±X %, as illustrated in FIG. 5, the charging is continuously performed at the constant charging current value c.